Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 171837

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
137 PM CDT Wed Jul 17 2019

1138 AM CDT

Main concern through early this evening is convective trends in
two separate areas of interest. First is the mature MCS
across central Iowa that high-res and short-term guidance has had
a universally poor handle on. Latest observational trends suggest
that northern portions of the linear MCS appear to be on a gradual
weakening trend. Meanwhile, the more intense portion of the linear
MCS looks to potentially bow more toward the southeast due to:
position of RIJ on latest radar mosaic; progged forward
propagating Corfidi vectors farther east; and existence of a
northwest to southeast instability gradient over the region due to
much steeper lapse rates farther west. It`s uncertain if a severe
threat will make it into portions of the CWA , however current
trends and extrapolated radar indicate that at least rain/thunder
have a decent chance to push into the western 1/3 or so of the CWA
toward and after 4pm. From the above thinking, most likely threat
area, if any, for strong winds with the MCS would be south of I-88
into north central and central IL.

Next area of interest is in the lake breeze convergence zone over
northeast/eastern IL and extreme western Indiana. SPC mesoanalysis
indicates that owing to mid-upper 80s temps and lower 70s Td,
we`re already uncapped at 1000-1500 j/kg MLCAPE. This is backed up
by the presence of solid vertical growth of the Cu field (noted on
webcams) in the vicinity of the lake breeze which has already
made a decent push inland. Despite fairly weak lapse rates, it
appears the lake breeze convergence should be sufficient for
widely scattered convection to develop and drift toward the east
and southeast. Isolated showers/storms will be possible as of this
writing, with CI over the Wisconsin state line up to Racine County
WI noted on latest radar mosaic. Based off satellite/radar trends
and solid CAM agreement in convection on the lake breeze, upped
PoPs into the chance range. Weak deep layer shear and weaker lapse
rates should preclude any severe threat with the lake breeze
activity, with the main threats lightning and heavy downpours,
though brief gusty winds would be possible.



304 AM CDT

For Today...

Barry`s remnant circulation continues to peel east of the region
early this morning. In the wake of this departing system, dewpoint
depressions are low across much of the area--generally running under
2-3 F outside of the urban corridors. Nighttime microphysics RGB
satellite imagery reveals low stratus percolating across our
eastern counties and surface observations are depicting ongoing
patchy visibility reductions. In addition, the corridor from near
and east of I-55 picked up pockets of heavier rainfall on Tuesday
afternoon, so this will be the region to watch for fog formation,
some of which could be locally dense. Farther to the west, mid-
level cloud cover associated with a weakening thunderstorm complex
should help curtail the potential for more widespread visibility
issues this morning.

Today is looking mostly dry as northern Illinois and northwest
Indiana will be situated along the backside of a departing mid-
level  shortwave. However, the combination of a lake breeze and
weak convergence associated with another nearby col may be enough
to spark isolated to perhaps widely scattered showers/storms this
afternoon in a very weakly capped environment. Forecast soundings
show convective inhibition eroding by late this morning and into
the early-afternoon hours. As a result, we`ve added some low (20%)
PoPs across parts of the area, mainly in a northwest-southeast
band near and west of the expected lake breeze boundary.

It will be warmer today compared to Wednesday with overall less
cloud cover (outside of diurnally-building cumulus and isolated
storms). Showing highs in the upper 80s and lower 90s, except
along  the immediate lakeshore where the aforementioned lake
breeze should get going pretty early today.

For Tonight...

Attention this evening and tonight turns upstream to a potential
thunderstorm complex which may bring a risk for strong to possibly
severe wind gusts to parts of the area. Guidance is in good
agreement that a burgeoning low-level jet (35-45 kts in the
925-850 mb layer) will intercept the northeastern periphery of a
plume of very steep mid-level lapse rates this evening to our
west. Associated warm advection and surface convergence in the
vicinity of a cold front should ignite a region of thunderstorms
at the nose of the LLJ...somewhere near the Iowa/Minnesota border.
Where this convective initiation occurs will be important, as
activity developing at this latitude will have a potential to dive
into our region as cold pool amalgamation occurs. The GFS
continues to be the north outlier with the LLJ, while CAMs are now
really keying in on the Iowa/Minnesota border region. Should a
sufficiently deep cold pool develop, a forward-propagating MCS
could subsequently move southeastward towards our area late this
evening and overnight.

Mid-level flow decreases with southward extent, and remains more
west-east oriented through Thursday morning. As a result, the
southern end of this complex may begin to gust out as it
approaches our CWA (and slows following upwind Corfidi Vectors),
while the eastern end continues to truck along more quickly to
the southeast. Still some questions regarding convective vigor by
the time this potential complex reaches Illinois, but a strong to
possibly severe wind threat will exist given the large reservoir
of instability. We will also have to monitor for training
potential at the SW end of this MCS as strong WAA will persist
through Thursday morning which could result in isolated flooding
issues, mainly in the upwind-propagating portion of the MCS likely
across our western counties.

For Thursday through Saturday...

***A period of dangerous heat is expected across the area
 Thursday through Saturday. Heat Advisories and Excessive Heat
 Watches have been posted. However, we want to stress that it`s
 not so important to focus on who is under an Excessive Heat
 Watch/Warning, or Advisory. The heat is forecast to be oppressive
 and dangerous everywhere, with possibly some of the hottest
 conditions since 2012.***

For Thursday morning, outflow from the potential overnight complex
may push through our entire CWA Thursday morning. As a result,
we`ve nudged morning temperatures and dewpoints down a bit. All
indications, however, continue to point to breezy southwest flow
enabling a quick airmass recovery through the afternoon hours.
Still some lingering questions about the degree of mixing on
Thursday afternoon and still think the near 80 degree dewpoints
from the NAM and GFS are a bit overcooked. The 03z RAP, which
tends to more aggressively mix due to the MYNN PBL scheme, hints
at a potential for dewpoints to drop into the lower 70s across
parts of the region Thursday afternoon. This will obviously have
implications for our heat index forecast.

Either way, Thursday will be the first day of a period of
dangerous heat across the region as highs build into the mid 90s
with dewpoints that should at least support heat indices of 105
across all of the region. The current forecast shows heat indices
of 106- 111 degrees for a widespread area roughly near and south
of I-88. Breezy southwest winds will help take the edge off a bit,

Guidance has been offering some hints that another MCS may try to
develop Thursday night across Michigan that could try to build
southwest into the low-level jet. At this time, think any precip
potential during this time frame will remain north and east of our
area, but we`ll continue to monitor trends in guidance.

Friday is still looking like the hottest of this upcoming stretch.
It`s not out of realm of possibility that places tag 100 degrees
during the afternoon (actual air temperatures). Heat indices of
106-112 are forecast area-wide. We also want to continue to stress
that sweltering conditions are anticipated overnight (both
Thursday and Friday) with lows likely only dipping into the upper
70s and possibly staying in the 80s in downtown Chicago. Record
high minimum temperatures may be in jeopardy at ORD and RFD.

Saturday continues to pose a conundrum for our temperature forecast.
Guidance has been waffling with the location of a cold front and
subsequent initiation of showers and thunderstorms. Obviously, if
storms manage to develop during the late-morning/early-afternoon
hours, temperatures could be quite a bit cooler than currently
advertised. That said, the latest guidance supports continued hot
conditions, especially for locales near and south of I-88 within
the thermal ridge. For locales north of I-88, we opted to hoist
a two-day Heat Advisory that does not include Saturday due to
slightly lower heat indices on Thursday and Friday and somewhat
higher potential for things to cool down Saturday afternoon with
the front. While heat indices south of I-88 are not explicitly
forecast to touch 110 Saturday afternoon, this would be the third
day of 105+ values over a widespread area, hence the three-day
Excessive Heat Watch for these locales.



308 AM CDT

Sunday through Tuesday...

Thankfully, relief from the heat does appear to be in sight come
Sunday as a sharp shortwave digs southward and helps finally send
a cold front into and eventually through the area. Shower and
thunderstorm chances will subsequently increase (as early as
Saturday evening). The mid-level flow increases a bit on Sunday
with the approach of this next shortwave, so there may be some
propensity for storm organization and an associated strong-severe
thunderstorm potential.

By Monday and Tuesday, temperatures should be back to near
seasonal normals in the lower to middle 80s along with markedly
lower dewpoints (in the 50s and 60s).



For the 18Z TAFs...

Multiple aviation concerns, including:
* Widely scattered TSRA on lake breeze through this afternoon
* Potential for impacts from area of TSRA later this afternoon-
  early evening, especially at RFD
* Another area of TSRA Thursday morning, with potential for
  direct TS impacts at terminals, including gusty winds & heavy
  rain/low VSBY
* Potential for wind shift to northeast/east behind TSRA Thu AM
* Southwest winds gusting to 20-25 kt Thursday afternoon
* Small chance for additional scattered TSRA Thu afternoon

SHRA/TSRA have started to pop up near the lake breeze early this
afternoon, with the highest coverage over southern WI, but an
increase in coverage to widely scattered expected over northeast
IL, covered by VCTS. Isolated cell near MDW did not have any
lightning yet but had added a tempo TSRA given close proximity.
Even SHRA without lightning should have convective cloud tops at
or above 30kft AGL. Will closely monitor trends for possibly
needing any additional TEMPO groups for brief direct impacts.
Otherwise, lake breeze with east and northeast winds will
gradually push inland, though outflow from TSRA could locally
modify the wind directions.

Attention then turns to the organized linear complex of
thunderstorms over eastern Iowa. The strongest portion of this
will likely surge southeast, with possible weakening north. It`s
uncertain whether precip from this complex will make it to the
eastern terminals, but did have enough confidence for TEMPO TSRA
at RFD late this afternoon. If SHRA and embedded TS do occur at
the eastern terminals, would most likely be 00z and later.

Yet another complex of TSRA is expected to affect the area
Thursday morning from the predawn hours near RFD and exiting east
of the eastern terminals by mid AM. Confidence is medium on
occurrence, but lower confidence on how strong the expected linear
complex will be when it moves across the area. Strong/severe gusts
would be possible if it remains intense. Would expect a gusty
west-northwest wind shift even if on a slow weakening trend,
followed by a brief period of northeast winds eastern terminals
and east- southeast near RFD behind the complex. Winds will come
around to southerly behind the morning variability and become
gusty, with southwest winds (210-230 direction) gusts to 20-25+ kt
through the afternoon. Soundings indicate little/no capping on
Thursday, so there is a non-zero chance of a couple TSRA popping
up during the afternoon, though confidence was too low for



IL...Heat Advisory...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008...noon
     Thursday to 11 PM Friday.

     Excessive Heat Watch...ILZ010-ILZ011-ILZ012-ILZ013-ILZ014-ILZ019-
     Thursday to 7 PM Saturday.

IN...Excessive Heat Watch...INZ001-INZ002-INZ010-INZ011-INZ019...noon
     Thursday to 7 PM Saturday.




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